In a motor vehicle, it is known to use a device for detecting the presence of a human hand on the handle of a door of the vehicle so as to unlock access to the passenger compartment thereof.
As is known, such a device comprises a sensor, installed at least partly in the handle, a converter and a microcontroller. During operation of the device, the sensor generates a voltage signal representative of the presence or of the absence of a hand on the handle, and the converter converts this analog signal into a digital value according to its amplitude, such that the microcontroller is able to detect the presence of a hand on the handle and unlock the passenger compartment.
In one known measurement solution, said to be “capacitive”, the sensor comprises a first capacitor, called “reference capacitor”, which is periodically charged and discharged into a second capacitor, called “storage capacitor”. When the reference capacitor discharges into the storage capacitor, the charges are balanced between the two capacitors. As is known, the analog voltage signal generated by the sensor is representative of the value of the voltage measured across the terminals of the storage capacitor when the charges are balanced.
When a human hand is present on the handle or close to the handle, for example less than 10 mm away, the level of charge of the reference capacitor increases. This results in a larger discharge of the reference capacitor into the storage capacitor, and therefore a higher level of balancing in the presence than in the absence of a hand on the handle.
The analog-to-digital converter (or ADC) makes it possible to convert the analog voltage measured by the sensor, representing the level of charge of the storage capacitor, into a coded digital value based on a reference voltage. In theory, each digital value corresponds, as is known, to a different analog voltage value interval but with a width equal to 1 LSB (“least significant bit”), the set of consecutive analog voltage value intervals extending over a range of voltage values whose width corresponds to the reference voltage of the converter.
As is known, coding on N bits makes it possible to use 2N digital coding values (or levels). By way of example and with reference to FIG. 1, coding may be performed on 3 bits. Such coding thus makes it possible to use 23, that is to say 8 digital values, defined by binary elements 000 to 111. In this example, the input voltage value V1 belongs to an analog voltage value interval corresponding to the digital coding value 100 in the example of FIG. 1.
The accuracy of the converter depends on the number of bits used to code the various digital values. Thus, the higher the number of bits, the more the resolution of the converter increases. A problem arises when the variation in the capacitance that it is desired to measure is low in comparison with the resolution of the converter. The result of this is that measured values that are far apart may be situated in the same analog voltage value interval and thus correspond to one and the same coded digital value, thus preventing any detection. There are several ways of rectifying this problem. A first known solution consists in increasing the number of coding bits of the converter, but this makes it more complex and more expensive, thereby exhibiting a drawback. A second known solution consists in oversampling the signal, which increases the resolution but requires significant processing capabilities, thereby exhibiting another drawback. In addition, a noise greater than 1 LSB is necessary at the input of the sensor in order to be able to use this oversampling technique, this not been the case in some types of sensor.
Moreover, in practice, it is generally observed that the resolution of the converter is not constant, this being reflected by the fact that the coded digital values generated by the converter correspond to analog voltage value intervals of different widths.
The difference in width between two consecutive intervals, as is known, is called differential non-linearity (or DNL) error. This differential non-linearity error is expressed in LSB. In theory, as mentioned above, the width of an analog voltage value interval corresponding to a coded digital value has a value of 1 LSB, but in practice this value varies by a difference, for example between more or less 1.5 LSB, which corresponds to the DNL error. Thus, a DNL error of more or less 1.5 LSB over all of the digital values leads to digital interval widths of between −0.5 LSB (which corresponds in practice to the digital value disappearing) and +2.5 LSB, that is to say two and a half times the theoretical width of an interval.
In order to detect the presence of a hand on a door handle of the vehicle with certainty, the measured difference between the digital values in the absence of a hand and the digital values obtained in the presence of a hand should be greater than a predetermined value. By way of example, the predetermined value representative of the presence of a hand may be 2 LSB, this value therefore representing a difference of two coded digital values.
With reference to FIG. 1, two analog voltage values V1 and V2 are representative of the absence and of the presence of a hand, respectively. In the theoretical case of a converter having a constant resolution, the two analog voltage values V1 and V2 belong to two different analog voltage value intervals corresponding to two different digital values (100 and 110 according to the example cited and presented in FIG. 1). As the two digital values are spaced by 2 LSB, this results in unlocking of the door of the vehicle.
Therefore, when the DNL error is large on a coded digital value, several input analog voltage values belonging to different analog voltage value intervals may correspond at output to this same digital value. In other words, when the predetermined difference for detecting a hand on the handle of the vehicle, for example 2 LSB, is less than the maximum value of the DNL error (for example 2.5 LSB), analog voltage values measured with or without the hand may have one and the same digital value at the output of the converter, thereby preventing detection of the hand on the handle and therefore exhibiting a major drawback. By way of example, with reference to FIG. 2, a converter may have a DNL error that leads to the occurrence of analog voltage intervals with a width of 2.5 LSB. Thus, output analog voltage values V1 and V2 from the sensor, which are identical to the example of FIG. 1, in this case correspond to one and the same output digital coding level (100 according to the cited example), whereas they correspond in theory to two digital values spaced by 2 LSB (100 and 110 in FIG. 1).
In the prior art, when the difference between two digital values is small, for example 2 LSB, it is possible to oversample the signal so as to increase the sensitivity of the system. However, such oversampling does not make it possible to solve the drawbacks linked to the DNL error. In addition, as explained above, noise is necessary at the input of the sensor in order to perform such sampling, of greater than 1 LSB, in order to detect a difference of 2 LSB, which is not the case in some sensors.